Silver-copper mixed oxides

Catch your nearest colleague and ask him/her the following question: do any silver-copper mixed oxides exist? Chances are that he/she will answer “yes”. Indeed, I would say that 99% of people would answer yes (I have actually confirmed it experimentally). This is not surprising if you think about the two metals separately, as they both form many different oxides, namely, Ag2O, AgO, Ag2O3, Ag3O4, Cu2O, CuO and Cu4O3. Both metals can even be found together in several compounds such as AgCuS, AgCuSe or AgCuPO4. But does a mixed oxide really exist? Well, as it turns out, it did not until 1999; before the twenty first century, not a single example of a mixed silver-copper oxide was known, neither natural nor synthetic. Mother Nature can be so whimsical! Indeed it was while E. Tejada-Rosales and P. Gómez-Romero were looking for a model compound in order to synthesize less toxic high temperature superconducting (HTSC) cuprates, by replacing Hg by Ag (a goal pursued by many labs around the world), that they realized no such mixed oxide existed. And, of course, off they went to try to synthesize the first member of the family, which they managed to achieve via a low temperature co-precipitation reaction [1]. Thus, 1999 saw the birth of the first silver-copper mixed oxide: Ag2Cu2O3. This material is isostructural with paramelaconite (Cu4O3, or Cu 2Cu2 2O3) with silver cations located at the Cu  positions. Soft chemistry synthesis methods, such as simple precipitation, have the advantage when dealing with silver oxides, as silver cations readily reduce to metallic silver when subjected to traditional solid state methods involving high temperatures (unless high pressures are also applied).

In 2000 I began my PhD with the objective of expanding the recently-created family of silver-copper mixed oxides in Prof. Casañ-Pastor's group (Materials Science Institute of Barcelona, ICMAB-CSIC). The strategy was again to use soft chemistry methods in order to avoid the reduction of silver and to utilize environmentally-friendly techniques. Thus, taking Ag2Cu2O3 as the starting point, we managed to oxidize the first Ag-Cu mixed oxide at room temperature both electrochemically and using ozone to obtain the next member in the family [2] and [3]: Ag2Cu2O4 (or AgCuO2). As the formula shows, a whole oxygen atom was intercalated per unit formula, giving rise to the new oxide. In this case, it has the crednerite structure (Cu Mn3 O2) with Ag cations located at the Cu  positions and Cu cations at the Mn3  sites. Yet, in spite of the crystallographic relationship with crednerite, the electronic structure of AgCuO2 was again different to what one would have thought. Instead of having Ag  and Cu3 , both cations are partially oxidized and the charge is delocalized among all the atoms in the oxide. The compound must therefore be described by the formula Ag(1 x) Cu(2 y) O2, where x and y depend on the synthesis method [4]. This unique electronic structure gives AgCuO2 a high conductivity [5] and [6]. Following this, the next Ag-Cu mixed oxide was synthesized using a low temperature hydrothermal reaction. The resulting Ag2CuMnO4 has the delafossite structure with Cu2  and Mn4  cations located in the B layer of the structure [7]. This compound shows complex magnetic behavior. While paramagnetic at room temperature, upon cooling it initially shows short range ferromagnetic interactions. With further cooling significant antiferromagnetic correlations also appear.

The image on the cover shows flat, hexagonal particles of yet another silver-copper mixed phase, which is currently the subject of study. These particles have been synthesized using a low temperature hydrothermal method. In many cases twinning is observed, as is often the case for materials obtained via hydrothermal reactions. Different twinned particles are clustered together evoking the sense of a desert rose. The image was obtained with a field emission scanning electron microscope and has been colored. With the discovery of each new member of this family, novel, interesting properties are found. I am currently in Prof. Driscoll's group (University of Cambridge) and one of my research interests is exploring the use of these and other inorganic compounds for energy conversion and storage applications.

Further Reading
[1] P. Gómez-Romero et al. Ang Chem Int Ed, 38 (1999), p. 524
[2] D. Muñoz-Rojas et al. Electrochem Comm, 4 (2002), p. 684
[3] D. Muñoz-Rojas et al. J Solid State Chem, 178 (2005), p. 295
[4] D. Muñoz-Rojas et al. J Phys Chem B, 109 (2005), p. 6193
[5] F. Sauvage et al. J Solid State Chem, 182 (2009), p. 374
[6] D. Muñoz-Rojas et al. Inorg Chem, 49 (2010), p. 10977
[7] D. Muñoz-Rojas et al. J Solid State Chem, 179 (2006), p. 3883